Depends on the level of clearance you're going for. TS/SCI w/ Full-Scope Polygraph (the highest intelligence community level) covers the last 10 years, with a heavy emphasis on your activities in the last 5. They'll interview friends, employers, teachers, neighbors, etc. going back 5 years (but you get to pick the ones to be interviewed). They'll also run FBI background checks on you and your family (including extended family) and you'll be required to turn in financial info annually.